This invention relates to a process for producing hydrocarbons useful as diesel fuel and aviation fuel from renewable feedstocks such as the triglycerides and free fatty acids found in materials such as plant oils, fish oils, animal fats, and greases. The process involves hydrogenation, deoxygenation (decarboxylation, decarbonylation, and/or hydrodeoxygenation) in at least a first zone and hydroisomerization and hydrocracking in at least a second zone. A selective hot high pressure hydrogen stripper is used to remove at least the carbon oxides from the hydrogenation, decarboxylation and/or hydrodeoxygenation zone effluent before entering the hydroisomerization and hydrocracking zone. Optionally, a diesel range stream, a naphtha range stream, a naphtha and LPG range stream, or any mixture thereof is used as an additional rectification agent in the selective hot high pressure hydrogen stripper to decrease the amount of product carried in the overhead thereby reducing the amount of n-paraffins in the diesel and aviation fuels.
As the demand for diesel fuel and aviation fuel increases worldwide there is increasing interest in sources other than petroleum crude oil for producing these fuels. One such source is what has been termed renewable feedstocks. These renewable feedstocks include, but are not limited to, plant oils such as corn, rapeseed, canola, soybean and algal oils, animal fats such as inedible tallow, fish oils and various waste streams such as yellow and brown greases and sewage sludge. The common feature of these feedstocks is that they are composed of triglycerides and Free Fatty Acids (FFA). Both of these compounds contain aliphatic carbon chains having from about 8 to about 24 carbon atoms. The aliphatic carbon chains in the triglycerides or FFAs can also be mono-, di- or poly-unsaturated. Some of the glycerides from the renewable sources may be monoglycerides or diglycerides instead of or in addition to the triglycerides.
There are reports in the art disclosing the production of hydrocarbons from oils. For example, U.S. Pat. No. 4,300,009 discloses the use of crystalline aluminosilicate zeolites to convert plant oils such as corn oil to hydrocarbons such as gasoline and chemicals such as para-xylene. U.S. Pat. No. 4,992,605 discloses the production of hydrocarbon products in the diesel boiling point range by hydroprocessing vegetable oils such as canola or sunflower oil. Finally, US 2004/0230085 A1 discloses a process for treating a hydrocarbon component of biological origin by hydrodeoxygenation followed by isomerization.
Described herein is a process which comprises one or more steps to hydrogenate, deoxygenate, isomerize and selectively hydrocrack a renewable feedstock in order to generate both a diesel range product and an aviation range product. Simply hydrogenating and deoxygenating the renewable feedstock in a hydrogen environment in the presence of a hydrotreating catalyst results in straight chain paraffins having chain-lengths similar to, or slightly shorter than, the fatty acid composition of the feedstock. With many feedstocks, this approach results in a fuel meeting the general specification for a diesel fuel, but not the specifications for an aviation fuel. The selective hydrocracking reaction reduces the carbon chain length to allow selectivity to aviation fuel range paraffins while minimizing lower molecular weight products. The volume ratio of recycle hydrocarbon to feedstock ranges from about 0.1:1 to about 8:1 and provides a mechanism to increase the hydrogen solubility and more uniformly distribute the heat of reaction in the reaction mixture. As a result of the recycle, some embodiments may use less reactor volume, less excess hydrogen, a lower operating pressure, or any combination of the above.
The performance of the isomerization and selective hydrocracking catalyst is improved by removing at least carbon dioxide from the feed to the isomerization and selective hydrocracking zone. The presence of oxygen containing molecules including water, carbon dioxide, and other carbon oxides may result in the deactivation of the isomerization catalyst. The oxygen containing molecules such as carbon dioxide, carbon monoxide and water are removed using a selective hot high pressure hydrogen stripper which optionally contains a rectification zone.